The present invention relates to an improved elastomeric copolymer of an aromatic vinyl compound and butadiene containing polymer chains with a molecular weight higher than a certain level in a specific proportion and being suitable for use in tires. More particularly, the invention relates to a rubber-like aromatic vinyl compound/butadiene copolymer having a glass transition temperature of not less than -50.degree. C., a specific proportion of the polymer chains of which have a molecular weight higher than a particular level and which has a low rolling resistance, a large resistance against a wet road (the resistance being hereinafter referred to as "wet grip") and an excellent processability.
In recent years, reduction of running fuel cost of automobiles is strongly demanded from the viewpoint of energy saving. Especially, it is of urgent necessity for the tire manufacturing industry to decrease the rolling resistance of tires, because the rolling resistance has an important influence of fuel saving.
Among the constituent elements of a tire, a tread rubber accounts for the largest proportion of the travelling resistance accompanied by rolling of the tire, and it is known that the tread rubber accounts for about 40% of the rolling resistance of the tire at large. The rolling resistance of a tire results from energy loss accompanied by repeated deformation of a running tire. Thus, the reduction of the rolling resistance is no more than reducing such an energy loss. Particularly, considering it with respect to the motion of a tread rubber, it is necessary to reduce energy loss due to a bending and shearing motion and a compressive motion. From the viewpoint of the dynamic viscoelastic characteristic of a rubber, this means reduction of the loss modulus (E") and the loss compliance [E"/(E*).sup.2 wherein E*=E'+iE", and E* is complex modulus, E' is storage modulus and i.sup.2 =-1]. Further, these motions are repeated motions attended on rotation of a tire, and the frequency thereof is from 10 to 10.sup.2 Hz. Accordingly, it is necessary for the reduction of the rolling resistance of tires to decrease the loss modulus and loss compliance of a tread rubber in 10 to 10.sup.2 Hz.
On the other hand, considering from the safety side in running of a car, the wet grip characteristic is an important characteristic required for tires. It is a characteristic concerning travelling performances such as steering stability and breaking performance on a wet road such as a road at the time of raining, and is a frictional resistance which is caused by contact of the tread surface of a tire with the road. In other words, since a tire shows a recovery from deformation with a time lag to a stress receiving from the road surface, a torque of a reverse direction to the running direction is generated. The resistance due to this torque is the frictional resistance, and is related to the loss coefficient (tan .delta.) from the viewpoint of the viscoelastic characteristic, provided that .delta. is a phase difference between stress and strain, and tan .delta.=E"/E'. Further, the frequency of the stress caused by the road is a very high frequency attended on the uneveness of road, and according to literature (e.g. Rubber Chemistry and Technology, Vol. 4, page 4), the frequency is from 10.sup.4 to 10.sup.7 Hz. Therefore, in order to improve the wet grip characteristic of tires, it is necessary to raise the loss coefficient of a tread rubber in 10.sup.4 to 10.sup.7 Hz.
In a tread rubber compound generally and widely used heretofore, natural rubber, polyisoprene rubber, high-cis-1,4-polybutadiene rubber and styrene-butadiene rubber are mainly used as a rubber component. The former three are superior in rolling resistance, but are poor in wet grip characteristic. On the other hand, a styrenebutadiene rubber is superior in wet grip characteristic, but is poor in rolling resistance. Therefore, a tread rubber composition having excellent both properties of the rolling resistance and the wet grip characteristic has not been obtained. A tread rubber composition has been used at the cost of either one property, or with keeping balance between the both properties by blending the above-mentioned elastomers. However, the present state is considered far from sufficient for the level demanded in recent years with regard to keeping a high wet grip characteristic and keeping a low rolling resistance at the same time.
The present inventors made a study of the preparation of a polymer having excellent processabilities such as kneading processability, roll processability and extrusion processability required in tire manufacturing as well as a fundamental study of the repugnant rolling and wet grip characteristics, and they found, as described in Japaneses Patent Application No. 53387/1982, that a polymer having a particular structure has a high wet grip characteristics and an excellent rolling resistance characteristics together with an excellent roll processability. The present inventors have continued the study in order to further improve the processabilities such as kneading processability, roll processability and extrusion processability required in tire manufacturing with maintaining a high wet grip characteristic and an excellent rolling resistance characteristic, and have now reached the present invention.
It is an object of the present invention to provide a rubber material suitable for use in the tread portion of tires.
A further object of the invention is to provide an elastomeric copolymer having improved processabilities required in tire manufacturing and capable of providing tires having excellent wet grip and rolling characteristics.
These and other objects of the present invention will become apparent from the description hereinafter.